This invention relates generally to a method for reducing the vulcanizing cycle time in the injection molding of rubber compounds and, in particular, in the injection molding of rubber compounds using ram type injection molding equipment. Rubber compounds for injection molding operations generally comprise rubber, an accelerator, a vulcanizing agent, fillers, and various other additives. It is generally known in the art that vulcanizing cycle time, or the time the rubber remains in the mold to complete the cure, is a limiting factor in rubber injection molding operations because it controls the rate of output of molded articles from the apparatus. A continuing objective therefore is to reduce vulcanizing cycle time so the apparatus can be more efficiently used.
In a typical rubber injection molding process, the uncured viscous rubber compound is introduced to the elongated barrel of an injection molding machine at ambient temperatures. It is advanced through the barrel toward a mold connected to the downstream end of the barrel, usually by either a rotating screw conveyor or a reciprocating ram or piston disposed in the barrel. As the compound advances, it is heated by conduction in the barrel in order to reduce its viscosity and render it more flowable and amenable for subsequent injection into the mold. The less viscous the compound, the more easily it flows through the runners and gates and the more easily it fills the mold to produce a satisfactory molded object.
Since curing of the rubber compound is a "time at temperature" phenomenon, the heating also serves to supply some of the time at temperature requirement in the barrel without prematurely scorching the compound in the barrel. This increase in temperature of course reduces the time at temperature required in the mold and consequently the vulcanization cycle time. As known in the art, most rubber compounds can be cured at either a shorter exposure to a higher temperature or a longer exposure to a lower temperature, and it is this phenomenon which is referred to herein by the term time at temperature.
Cure time in ram injection rubber molding, for example, consists of three separate and distinct time at temperature periods. The first is the time at temperature during the compounding and storage of the material prior to entering the barrel of the injection molding apparatus and is referred to as the "process scorch time". The second time at temperature is the "residual scorch time" attained or permitted in the barrel of the apparatus. The higher the time at temperature during the process scorch time, the lower will be the time at temperature for residual scorch time in the barrel of the apparatus. The third time at temperature is the "vulcanization time" of the compound within the mold itself. The three periods of time together comprise cure time and the nature and degree of the time at temperature of the first two have an effect upon the third, vulcanizing time. Thus, a rubber compound that has been exposed to a higher time at temperature relationship during process time or during its passage through the barrel will vulcanize more quickly than a compound that has been exposed to lower conditions of time at temperature. Thus rubber compounds at higher temperatures will vulcanize more quickly than rubber compounds at a lower temperature. In most injection molding operations, a smaller portion of the time at temperature requirement is supplied in the barrel of the injection molding apparatus (the residual scorch time), and a larger part in the heated mold (the vulcanization time).
In addition to the cumulative effect of the above time at temperature periods, there is a critical temperature range for each rubber compound called the "critical residual scorch temperature range" at which the vulcanizing of rubber is initiated. These temperatures ranges are known to those skilled in the art. For the rubber compound used to obtain the data in Table I below, the critical scorch temperature range was 160.degree.-170.degree. F. Just above that temperature range the compound will begin to "scorch" or vulcanize in some period of time, which may be minutes or seconds Just below that temperature range vulcanization may require hours.
In a typical rubber injection molding process the objective is to heat the rubber compound to the maximum temperature, just below this critical scorch temperature range, which will produce the lowest viscosity of the compound at this limited temperature. The inability to supply more temperature or heat energy or time at temperature in the barrel so that vulcanization time in the mold can be reduced has been a continuous problem in prior art processes and apparatus. It is toward this problem that the present invention is generally directed.
The rubber compound is usually heated by externally heating the barrel of the apparatus electrically, with a steam jacket or from some other such external heat source, and transferrring the heat by conduction from the hot barrel wall into the mass of the rubber compound moving through the barrel. Some additional heat is usually supplied to the compound by frictional forces and by shearing of the rubber compound which occurs in the sprue, runners and gate system of the mold and, in many cases, this additional heat is an important factor depended upon for vulcanization. Once in the mole, additional heat is supplied to the compound and the compound is held in the mole for the required time at temperature for vulcanizing end to complete the cure.
Vulcanizing cycle time could be reduced if the compound could be rapidly and uniformly heated in the barrel to a higher temperature and then quickly injected into the mold so that more of the time at temperature required to cure the rubber compound had occurred before the compound entered the mold. However, the rubber compound cannot be exposed to high temperatures for even short periods of time in the barrel or undesirable scorching would take place before the rubber compound even entered the mold. One difficulty encountered in attempting to quickly and uniformly heat the compound, while it is still in the barrel, stems from the poor thermal conductivity of the compound. This makes it difficult to use external heat to quickly heat the compound to a uniform temperature throughout. To rapidly obtain the desired temperature in the portions of the compound distant from the heat source, e.g. the electricity or steam heated barrel wall, it is necessary for the heat source to have a temperature substantially above that desired in the compound. This produces local hotspots in the compound in proximity to the barrel wall which cause formation of an undesirable skin of scorched compound or prematurely vulcanized rubber compound near the barrel wall. This can produce undesirable pieces of cured compound in the material before it even reaches the mold for final curing of the rest of the product. These pieces can clog the sprue and mold runners and ruin the molded product. As a result, the temperature of the barrel wall is usually maintained sufficiently low to avoid such hot spots and is kept below the critical scorch temperature. Consequently, the compound temperature does not become excessively high so that only a relatively small portion of the time at temperature required to cure the rubber compound is provided in the barrel. Furthermore, the temperature of the compound varies throughout, with the compound more distant from the barrel wall cooler than that close to the wall. The result of these factors is that a longer vulcanization cycle is required once the compound is in the mold in order to provide the time at temperature required to complete the cure of the entire mass of material.
Various techniques have been proposed to more quickly and uniformly heat the compound entering the mold to high temperatures in the barrel. In one approach, a torpedo mounted to the barrel, nozzle, rotating shaft or some other portion of the apparatus is disposed in the barrel to form a thin annular channel between the torpedo and barrel wall. The barrel wall surrounding the thin channel is heated. Since only a thin film of compound is being heated, quicker and more uniform heating of the compound is obtained, although it is still considered necessary to preheat the compound upstream of the torpedo, with the torpedo heating step being but a supplementary heating of the material. There still remains, however, some variation between the temperature of the compound next to the barrel wall and that closer to the torpedo. To increase the heat transfer into the compound, it has been proposed to heat the torpedo as well as the barrel wall.
The heating of the compound increases its temperature and reduces its viscosity to produce a heated plasticized more flowable material suitable for injection into the mold. However, since the temperature of the injected material is not uniform, and since the time at temperature in the barrel is well below that required to cure the compound, relatively long vulcanizing cycles in the mold are still required. Other techniques for controlled heating of the compound have also been suggested such as high frequency or micro-wave heating.
Proposals have been made to utilize the energy released when viscous materials are mechanically worked under conditions of shear to heat plastics for use in either extrusion or injection molding processes. For example, U.S. Pat. Nos. 3,351,694 and 3,488,416 mentioned a plasticizing-extruder commonly known as the "elastic melt extruder" which utilizes for heating the normal force developed when a viscous material is sheared in a gap between a rotating plate and a stationary plate. U.S. Pat. No. 2,668,986 mentions the shear which occurs in the film of plastic passing through the gap between a rotating torpedo and the barrel wall of a screw type extruder or injection molding apparatus, although the barrel must still be externally heated. It is also known that heat is generated by the shear working which takes place in a screw conveyor injection molding apparatus or extruder although external heating of the barrel is usually also required, see, for example, U.S. Pat. No. 3,467,743 and "Ram v. Plunger Screw in Injection Molding," L. W. Meyer et al., Plastics Technology, pp. 39-45 (July 1962). The adiabatic operation of a screw type thermoplastic extruder has been proposed in which substantially all the heat arises from the mechanical working of the plastics through viscous shearing, see "Adiabatic Extrusion of Polyethylene," J. M. McKelvey et al., SPE Journal, pp. 22-30 (March 1954), and Plastics Extrusion Technology and Theory, Gerhard Schenkel, American Elsevier Publishing Co., Inc., New York, pp. 55-63 (1966). However, insofar as is known, no entirely satisfactory solution to the problem of more uniformly and quickly heating rubber compounds in a ram type injection molding apparatus has been proposed in which vulcanization cycle time is substantially reduced because more of the time at temperature requirement of the compound takes place in the barrel before the compound enters the mold, but in such manner that the compound is not prematurely vulcanized in the barrel.
It is therefore an object of this invention to provide a method by which rubber compounds can be quickly and uniformly heated to a substantially homogeneous relatively high temperature above the critical scorch range without supplying heat to the compound by conduction from an external heat source, whereby a substantial portion of the time at temperature requirement for curing the rubber takes place before the rubber compound enters the mold cavity so that vulcanization cycle times in the mold are substantially reduced.
It is another object of this invention to provide a method by which a highly viscous rubber compound is sheared in the barrel of a ram type rubber injection molding apparatus to thereby substantially lower its viscosity and autogenously increase its temperature, without substantially increasing the residence time of the compound in the barrel and without supplying heat to the compound by conduction from the barrel wall, to thereby produce a compound heated above its critical scorch temperature and having a substantially uniform temperature throughout which can be injected into a mold and quickly vulcanized therein.
It is another object of this invention to provide and method for forming in the barrel of a ram type rubber injection molding apparatus a substantially less viscous rubber compound than heretofore obtainable in such equipment which, because of its low viscosity, more completely fills the contours of the mold to produce precision molded objects having a smoother and more glossy surface than heretofore obtainable.
It is another object of this invention to provide a method for forming in the barrel of a ram type rubber injection molding apparatus a substantially less viscous rubber compound than heretofore obtainable in such equipment whereby gases entrapped in the compound readily escape in the mold.
It is another object of this invention to provide a method for forming in the barrel of a ram type rubber injection molding apparatus a substantially less viscous rubber compound than heretofore obtainable in such equipment whereby highly viscous compounds and compounds loaded with large amounts of fillers, which in many cases are processed in conventional equipment only with great difficulty, or if at all, are readily injected into the mold to form acceptable molded parts.
It is another object of this invention to provide a method for injection molding rubber compounds in a ram type rubber injection molding apparatus with reduced vulcanization cycle times in which no heat is supplied to the compound in the barrel by conduction from an external source thereby permitting shorter barrel length and shorter retention times in the barrel than are normally required in apparatus known to the art.
It is another object of this invention to provide a method which largely eliminates the dependence on the temperature rise of the rubber compound which occurs in the runners and gate systems of prior art apparatus.
These and other objects of the invention will be apparent to those skilled in the art from a consideration of this entire disclosure.